We have examined the nucleotide sequences of a series of murine antibody genes derived from one kappa light chain gene in order to gain insight into the mechanism that specifically mutates variable genes. Six rearranged VK167 genes from hybridoma and myeloma cells were cloned from lambdaphage libraries. The sequences were compared to the germ-line sequence of the VK167 gene, the JK genes, and the CK gene to identify sites of mutation. Four out of six rearranged genes have extensive mutation, which occurred exclusively in a one kb region of DNA centered around the V-J gene. No mutations were found at more distant sites in the intervening sequence or in the constant gene. The frequency of mutation was approximately 0.5% (32 mutations/6749 bp). Mutations were mostly due to nucleotide substitutions with no preference for transitions or transversions. The location of mutations around each gene indicates that they occur in clusters at random sites. The observation of mutations in the intervening sequence downstream from the JK5 gene rules out models for the mechanism of mutagenesis that rely solely on gene conversion or recombination. The distribution and high frequency of mutations is most easily explained by a mechanism of error-prone repair that occurs during several cycles of cell division.